In medical imaging, accurate diagnosis depends on the optimal presentation of the image, either on hardcopy or softcopy display. The optimal presentation will allow the radiologist to observe and detect small abnormalities that may not be visible in a presentation that is less than optimal.
In conventional screen/film radiography, the tonescale curve is built into the film as a function of the emulsion developed by the film manufacturer. The chemical development of the film also affects the final image visible on the film. Different films are available from manufactures to obtain "different looks" on the film.
The process of digitizing a film or obtaining the image by digital means, either storage phosphor or direct digital techniques, separates the acquisition step from the imaging processing steps used to obtain the final image. This separation allows an arbitrary tonescale curve to be used to obtain the final look of the image.
The purpose of the tonescale curve is to map the relevant code values obtained from either the digitization process or the direct digital acquisition to the final range of code values that will result in an optimal image presentation in some sense. Typically, this would not be a one to one mapping, and certain ranges of code values would be allocated more dynamic range than other code values in the final rendered image.
A common approach to find the region of interest is to analyze the code value histogram. This method works best when the peaks in the histogram corresponding to the region of interest are separated from the undesired regions. If the regions overlap, it becomes more difficult to find the region based on the analysis of the histogram only. Goodenough et al., U.S. Pat. No. 5,068,788, issued Nov. 26, 1991; Namiki et al., U.S. Pat. No. 5,198,669, issued Mar. 3, 1993; Doi et al., U.S. Pat. No. 4,839,807, issued 4,839,807, issued Jun. 13, 1989; Gouge, U.S. Pat. No. 5,040,225, issued Aug. 13, 1991; Shimura, U.S. Pat. No. 4,914,295, issued Apr. 3, 1990; Tanaka, U.S. Pat. No. 4,952,805, issued Aug. 28, 1990, are all histogram based techniques that do not solve this problem.
A method has been proposed by several researchers to obtain an optimal image by first segmenting the body part from foreground and background regions and then performing a histogram analysis on the remaining segmented image (Capozzi and Schaetzing, issued Nov. 17, 1992, U.S. Pat. No. 5,164,993; Jang and Schaetzing, issued Dec. 7, 1993, U.S. Pat. No. 5,268,967). These techniques require successful separation of the body part from the foreground and background.
A method based on the texture analysis of the image is described by Gaborsid, et al., U.S. Pat. No. 5,426,684, issued Jun. 20, 1995. Although this method overcomes some of the short comings of analyzing the code value histogram itself, the method requires substantial computer time to calculate the texture features.
All of these techniques find the tonescale curve based on either global code values or global bone and tissue regions. None of the methods use spatially located regions of interest.